The treatment of food products by the application of electrical energy is by no means a new endeavor. In recent years, however, it has undergone somewhat of a renaissance due to a number of obtainable advantages. Electrical based process techniques can offer rapid heating times, high temperature changes and very uniform efficient energy application. However, the known techniques generally suffer from shortcomings which have limited their desirability and utility in food product treatment.
Principal amongst the difficulties associated with the electrical treatment of food products is the resulting electrochemical process of electrolysis. One form of electrolysis results in the dissolution of the metal electrodes inserted into the food product being treated. As ions flow between the pair of electrodes, the metal at the surface of the electrodes becomes ionized by releasing electrons to positive ions in the solution. The metal ions then dissolve into the food product being treated.
Another electroheating problem is caused by the conversion of conductive ionic species within the food product being treated to radicals and molecules, such as the conversion of a hydrogen ion to hydrogen atoms or gas and chlorine ions to chlorine atoms or chlorine gas molecules. Hydroxide ions can form hydroxyl free radicals and subsequently can be converted to water and oxygen. These conversions can adversely impact the flavor and other advantageous qualities of the treated food product both as a result of the direct depletion of ions and their subsequent conversion to other species and by initiating other reactions within the food product such as oxidation and reduction.
The food science industry has recognized the electroheating problems associated with the electrical treatment of food products and has attempted to respond in a number of ways. For example, U.S. Pat. No. 4,739,140 (the "'140 patent") describes an electroheating technique using high frequency AC electric current. The inventor, (also the present inventor), found that when frequencies exceeding typical household frequency, and in particular, exceeding 100 kHz are applied through conventional metal electrodes, both forms of electrolysis, i.e. dissolution of the electrodes and conversion of, for example, ions to gas molecules, are avoided. While this technology offers the clear advantages of eliminating electrolysis in the heat treatment of food products, it nonetheless requires the use of expensive equipment to generate the high frequency alternating current. Some food product processors would consider the capital expenditures necessary for such equipment to be prohibitive.
U.S. Pat. No. 4,695,472, (the "'472 patent") discusses methods and apparatus for extending the refrigerated shelf life of fluid food products including eggs. The technique involves the repeated application of high voltage, high current density, discrete electric pulses to the food products. Field strengths used are, at a minimum, 5,000 volts/cm and voltages as high as, for example, 37,128 volts are disclosed. Direct current densities of at least about 12 amps/cm.sup.2 are also disclosed as are pulse frequencies of between 0.1 and 100. Preferably, the treatment involves the application of at least 2 and, more preferably, at least 5 discrete high energy pulses to the food product being treated.
The '472 patent suggests that different forms of energy, i.e. the application of different types of pulses, can result in different effects on the treated food product. For example, the '472 patent discloses the use of flat-top electric field pulses where heating is to be kept to a minimum and exponentially decaying pulses where heating by high energy electric field is beneficial.
The '472 patent recognizes the problems associated with electrolysis and suggests a device which purports to eliminate these problems. The device optionally includes plate electrodes separated from the food product being treated by a membrane. An electrolyte is placed between the membrane and the electrode such that the effects of electrolysis, if any, are limited to the electrolyte and do not impact the food product being treated. Apparently, this technology was not principally designed to heat food products so as to cause the destruction of bacteria, but rather to utilize electrical pulses, often in combination with heat, and their destructive nature directly. This is evident by the disclosed material used to construct the electrode cell, and the membrane in particular. It has been found that the materials disclosed as useful membranes generally would not be useful to all electrical applications. In particular, it is believed that such membranes could not withstand the heat generated by the continuous flow of some forms of electrical energy, particularly over long periods of time.
In addition, while the use of pulses to effect an increase in extended refrigerated shelf life is disclosed, the '472 patent does not teach or suggest electroheating as described in the '140 patent or herein. For example, in treating liquid whole egg containing preservatives, the '472 patent exemplifies attaining a temperature of about 136.degree. F. This is significantly below the temperatures recognized by the United States government as necessary for thermally pasteurizing liquid whole egg. Nevertheless, extended refrigerated shelf life was realized. Clearly, therefore, the '427 patent is not relying on a thermal based method of pasteurization.
Another approach offered by APV U.K. Ltd. is the use of rare metals such as gold or platinum or other exotic alloys to construct or coat the electrodes. These particular materials do not exhibit the dissolution associated with more common materials such as stainless steel. See U.S. Pat. No. 4,959,525. On the other hand, this solution ignores other aspects of electrolysis previously described which also effect the flavor and quality of the food product being treated.
An electroheating apparatus using metal electrodes in direct contact with food products other than liquid egg products using AC voltage of less than 650 volts and a frequency of about 50 Hz is known from International Application No. PCT/GB88/00566. As a result of the low frequency and the direct contact of the food product with the metal electrodes, it is expected that the metal electrodes through electrolysis will slowly dissolve and contaminate the treated food product. It is precisely this problem of dissolution of the electrodes that was overcome by the '140 patent. In addition, the International Application recognizes that the food product being heat treated will ultimately bake upon the electrode surface. This necessitates the use of a scraper assembly to periodically scrape the interior of the electrode apparatus and the electrode surfaces. Obviously, any electroheating apparatus which is known to cause fouling of the electrodes by detrimental baking of the food product, is not suitable for use with delicate heat sensitive food products such as liquid egg products and the like.
Thus, for one reason or another, all of the potential solutions known to date are either incomplete, economically prohibitive or inconvenient to implement. The present invention provides for an inexpensive alternative to the use of high frequency devices such as those described in the '140 patent and/or high voltage-high energy pulse devices such as those described in the '472 patent. At the same time, the present invention affords complete protection from electrolysis, whether or not rare earth metal electrodes are used.
It has been discovered that with the right processing safeguards, relatively low frequency electrical energy (less than 100 kHz and preferably less than 100 Hz) having a sinusoidal wave form and, in particular, typical mains frequency (i.e., about 60 Hz in the United States, 50 Hz in Europe), can be drawn straight from a conventional outlet and used to electroheat food products. This can be accomplished without excessive cost, inconvenience, or concern over electrolysis.